1. Field of the Invention
This invention relates to devices which interlock electrical switches to preclude two switches from being in the "on" position simultaneously or to lock out a single switch.
2. Background of Information
There are a number of applications where it is required that the operation of two electrical switches be coordinated such that only one switch can be in the "on" position at a time. One such application is the transfer switch which selectively provides a load with electrical power from either of two different sources, such as for example, a commercial power system and an auxiliary supply which may be a diesel generator, or even another commercial source. Often circuit breakers are used as the switches in such transfer switches although switches without overcurrent protection are also used. It is imperative in such transfer switches that in transferring between independent sources that the switch disconnecting the former source be turned to the "off" position before the switch connecting the new source is turned to the "on" position to preclude interconnecting two sources with a random phase relationship.
Another application for interlocks is in ac motor control circuits such as reversing controls where one switch is used to connect the motor to a source with one phase rotation for forward operation and another switch connects the motor with the opposite phase rotation for reverse operation. Here to, the switches, which may be contactors, motor starters, motor controllers or switches without overcurrent protection can not connect both of the sources to the load at the same time. As used throughout, the term switch will be understood to refer to any of the above mentioned or similar types of switches used in applications where the operation of multiple switches must be coordinated by interlocks. There are some applications where it is desirable to interlock a single such switch.
Some interlocks couple the handles of switches whose operation is to be coordinated. In another type of interlock, used for instance especially when the switches are circuit breakers, has a plunger mounted in the switch housing of each switch which when actuated engages the switch operating mechanism to prevent the switch from closing. In one such interlock, the plungers are coupled to opposite ends of a walking beam so that when one switch is closed it pushes down on its plunger thereby pivoting the walking beam and raising the other plunger to block closing of the other switch. The circuit breaker which is held open lacks sufficient force to override the interlock and force the closed switch open. However, application of a force attempting to close the switch blocked open, places a strain, once the free travel of the handle is taken up, on the mechanism all the way back to the plunger of the switch which is on and the components must be robust enough and constrained sufficiently that they cannot be distorted to the extent that the interlock function is defeated.
In addition, the walking beam type of interlock requires that the plungers on the two switches be aligned in the same plane. This is difficult in some instances because of the arrangement of buses which connect the switch to the line side of the source and to the load may intrude into the alignment plane. Also, it is designed for switches placed side-by-side, but in some installations the switches may not be so aligned.
Another type of interlock utilizing plungers which engage the internal operating mechanisms of the switches is disclosed in U.S. Pat. No. 4,286,242. In this interlock which is designed for use with SPB type circuit breakers, the plunger of a first circuit breaker to be closed engages a lever on a rod to rotate the rod about its axis. Another lever on the other end of the rod engages a push rod which holds the second circuit breaker in the trip-free condition. An identical mechanism engaged when the second circuit breaker is closed, holds the first circuit breaker in the trip-free condition. Thus, two complete mechanisms are required. Again, any attempt to close one circuit breaker while the other is closed, returns the open breaker to the trip-free condition. This interlock requires that the circuit breakers be aligned end-to-end.
There is a need therefore, for an improved interlock for coordinating the operation of two switches.
There is a need for such an improved interlock in which forces generated by an attempt to close a switch which is locked open are not transmitted through the entire mechanism.
There is also a need for such an improved interlock which does not require that the switches be located side-by-side.
There is an additional need for such an improved interlock which provides flexibility for various arrangements of bus bars connected to the switches.